This invention relates to methods for applying a protective film to an integrated circuit; and more particularly to methods for applying a protective film to a dynamic memory integrated circuit for the purpose of protecting that memory against alpha-particle radiation.
In a dynamic memory integrated circuit, each memory cell is comprised of a capacitor for storing charges that represent the information in the cell. Typically, this capacitor is formed by the combination of a selected surface region of a semiconductor substrate, an insulating layer lying on that region, and a conductive layer lying on the insulating layer. With that structure, the selected surface region of the semiconductor substrate and the conductive layer over it form two parallel plates of the capacitor.
Typically, thousands of such capacitors are formed in a single substrate of which the total area is less than 1/10 of a square inch; and they are interconnected in an array by conductive lines and transistors. These lines and transistors generally are only 1-5 micrometers wide. Then, in operation, the charges representing information are selectively read from and written into the various capacitors.
Over the past several years the tendency in the dynamic memory art has been to continually increase the number of memory cells that are fabricated in a single semiconductor substrate. To a large extent this has been achieved by continually reducing the size of each individual capacitor. But as the size of each capacitor is decreased, the amount of charge which it can hold also decreases. Consequently, small quantities of charge, such as those generated at random in the semiconductor substrate by alpha-particle radiation, can combine with and offset the charges which are normally in the capacitor representing information. And this results in what is known as "soft errors".
To overcome this problem, protective films have been attached to the surface of the dynamic memory. These protective films act as barriers to the alpha particles, and thus they prevent the generation of unwanted charges in the semiconductor substrate.
In the prior art, the film was attached to the memory by applying a glue to the memory's surface; and thereafter the film was pressed, either manually or by machines, against the glue in order to cause sticking and bonding. But this pressing step is a very delicate operation; because if too much force is used, any one of the thin interconnections in the memory will fracture. And that makes the entire memory useless.
Also, the film must be aligned very accurately with the surface of the memory to insure that it covers all the memory cells. Otherwise, if the film is misaligned such that some of the memory cells are exposed, those cells would be susceptible to alpha-particle radiation. But this alignment step is difficult to perform accurately because the memories are so small.
In the prior art, the above attaching and aligning steps were performed either by machinery that was highly accurate, or by technicians that were highly skilled. But this solution for the problem is undesirably expensive.
Accordingly, a primary object of the invention is to provide an improved method of attaching a protective film to an integrated circuit.